A polarizing plate for a flat panel display, especially a polarizing plate used in LCD, generally employs a polyvinyl alcohol (PVA)-based film with a thickness of 70 μm or more mainly as a raw material. In order to obtain sufficient optical characteristics as LCD, preferably used is a polarizing plate manufactured by stretching a PVA-based film containing a dichroic material such as iodine and adhering a transparent protective film onto the stretched PVA based film. The polyvinyl alcohol-based polarizer is stretched in its manufacture; therefore the polarizer is easy to shrink. Since a PVA-based film is made of a hydrophilic polymer, the film is very easily deformed especially in a humidified condition. Moreover, since a mechanical strength of the film itself is low, the film has a problem of tearing. Hence, transparent protective films such as a film made of saponified triacetyl cellulose are adhered onto both sides or one side of a polarizer to thereby supplement its mechanical strength for its actual use. The polarizing plate is manufactured by adhering a transparent protective film to a polarizer with an adhesive. An aqueous adhesive is preferably used as an adhesive for a polarizing plate used in adhering a transparent protective film to a polarizer: an example thereof is a polyvinyl alcohol-based adhesive obtained by mixing a crosslinking agent into a polyvinyl alcohol aqueous solution.
Improvement on uniformity and quality of a screen image has been demanded in company with progress in high definition and high functionality of LCD. A heat resistance, a wet heat resistance and a water resistance at high levels have been requested because of a trend to more of diversification in usage environment. A low profile and weight reduction of LCD have also been required because of a trend to a function of portability in recent years. Requirements directed to a polarizing plate for LCD, from such required characteristics of LCD, include: higher performance and higher functionality as a polarizer at various angles such as uniformity, high heat resistance, high wet heat resistance, water resistance, low profile and the like.
A method for manufacturing a polarizing plate by adhering a transparent protective film onto a polarizer is exemplified as follows: First of all, as one method, an adhesive solution is coated on one surface of the polarizer and thereafter, a transparent protective film is adhered to the one surface. Then, the adhesive solution is coated onto the other surface of the polarizer and thereafter, a transparent protective film is adhered to the other surface thereof. Thereby, the transparent protective films are adhered to both surfaces of the polarizer with the adhesive layers interposed therebetween to thereby manufacture a polarizing plate. Note that the adhesive solution is coated directly on the transparent protective film or the adhesive solution is coated on both of the polarizer and the transparent protective film. Another method has been proposed in which, transparent protective films are disposed on both surfaces of a polarizer and the transparent protective films and the polarizer are caused to continuously pass through between rolls in a pair while an adhesive solution is supplied to therebetween (see JP-A No. 11-179871), or alternatively, transparent protective films and a polarizer are caused to continuously pass through between rolls in a pair in a state where adhesive layers are formed by coating an adhesive on surfaces of the transparent protective films on each of which adhesive layers are formed or/and an adhesive layer is formed by coating an adhesive on a surface of a polarizer on which the adhesive layer is formed, and the transparent protective films are adhered to the polarizer by a pressure of the rolls, thereby manufacturing a polarizing plate.
In the conventional process, however, a problem has been arisen that stripe-shaped appearance faults are generated during adhesion of the transparent protective films and the polarizer. Such a stripe-shaped appearance faults exerts an adverse influence on required high characteristics of a polarizing plate, especially on an optical uniformity among them. Therefore, there has been a reality that a progress in the related technology cannot cope with improvement on uniformity and quality of a screen image required in company with progress toward higher definition and higher functionality of LCD. The term “stripe-shaped appearance faults” means a view of in the absorption axis direction of a polarizing plate obtained by adhering a transparent protective film thereto, when being visually observed, as a reflection image of stripes parallel to one another. A feature of the stripes is to be recognized as track grooves cut on a phonograph record with a pitch in the range of 1 to 2 mm.
A polarizer is used as a polarizing plate reinforced by a transparent protective film in strength. A case has been conventionally arisen, however, where a polyvinyl alcohol-based adhesive causes separation at the interface between the polarizer and the transparent protective film in a humidified condition. A possibility can be assumed as a cause thereof that a polyvinyl alcohol-based resin, which is a main component of the adhesive, is a water-soluble polymer, suffers dissolution thereof into water of dew condensates.
In order to solve such a problem, a resin solution including a polyvinyl alcohol-based resin having an acetoacetyl group therein and a crosslinking agent is employed as a PVA-based adhesive to thereby improve wet heat resistance and water resistance (see JP-A No. 7-198945). However, a wafer resistance of an adhesive for a polarizing plate, which is described in an embodiment in JP-A No. 7-198945., has been insufficient. Moreover, the adhesive has been insufficient because of stripe-shaped appearance faults.
A method has been proposed in which an adhesive layer is provided on a polarizer or a transparent protective film and a contact angle of the adhesive layer is previously controlled at a predetermined angle or less by humidification and thereafter, the polarizer and the transparent protective film is adhered to each other to thereby manufacture a polarizing plate (see JP-A Nos. 7-306315 and 7-306316). In the methods of JP-A Nos. 7-306315 and 7-306316, a control is difficult and a fabrication process is complicated; therefore, the methods cannot be said to be practical manufacturing methods.